The MainContext that miscellaneous session-related asynchronous callbacks are invoked on.
(Eg, setting idle_timeout will add a timeout source on this context.)
For a plain Session, this property is always set to the MainContext
that is the thread-default at the time the session was created, and cannot be overridden. For the deprecated
Session subclasses, the default value is null, meaning to use the global
default MainContext.
If use_thread_context is false, this context will also
be used for asynchronous HTTP I/O.